Vertical slice architecture is an approach for organising your code into features/vertical slices rather than organising by technical conerns (e.g. Controllers, Models, Services etc). Each slice will contain all the code which fullfills a feature or use case inside the application. One of the main benefits of VSA is the ability to structure each feature/slice independently, so each feature/slice can be as simple or complicated as it needs to be.

Both Onion/Clean Architecture and Vertical Slicing Architecture aim to structure applications for better maintainability, scalability, and testability, but they take different approaches in organizing the codebase. Here's a breakdown of their differences:

✅ Layered Architecture
✅ Focuses on Separation of Concerns
✅ Domain-Centric (Business Logic First)
✅ Code is Organized by Layer (API, Application, Domain, Infrastructure)

/TaskManager
│── /API                 --> Presentation Layer (Controllers, Endpoints)
│── /Application         --> Business Logic (Use Cases, DTOs, Services)
│── /Domain             --> Core Business Rules (Entities, Aggregates)
│── /Infrastructure     --> Persistence, External APIs, Logging

• The Domain is at the center (contains entities and business rules).
• The Application Layer contains use cases, DTOs, and services.
• The Infrastructure Layer includes database, logging, and external integrations.
• The API Layer is just an interface for external consumers (Controllers/Minimal APIs).
• Dependencies flow inwards (API → Application → Domain).

✔️ Strong separation of concerns and modular structure.
✔️ Testability (business logic is independent of frameworks).
✔️ Scalability (supports different UIs like Web, Mobile, or gRPC).

❌ Can be over-engineered for simple projects.
❌ Slower to develop due to multiple layers.

✅ Feature-Based Organization
✅ Each Feature is Independent
✅ Reduces Unnecessary Abstraction
✅ Follows CQRS and Mediator Patterns

/TaskManager
│── /Features
│    ├── /Tasks
│    │    ├── /Create
│    │    │    ├── CreateTaskHandler.cs
│    │    │    ├── CreateTaskRequest.cs
│    │    │    ├── CreateTaskResponse.cs
│    │    ├── /GetAll
│    │    │    ├── GetAllTasksHandler.cs
│    │    │    ├── GetAllTasksRequest.cs
│    │    │    ├── GetAllTasksResponse.cs
│── /Infrastructure      --> Shared Infrastructure (DB, Logging)
│── /API                 --> Entry Point (Minimal API, Controllers)

• Each feature (e.g., Tasks) is self-contained with its own:
  • Handler (Processing logic)
  • Request DTO
  • Response DTO
• Uses CQRS (Command Query Responsibility Segregation) and MediatR for clean request handling.
• No unnecessary abstraction (like repositories or service layers).

✔️ Faster Development (each feature is independent).
✔️ Avoids unneeded complexity (no forced service/repository layers).
✔️ Easier to scale individual features.
✔️ Better for Microservices (as features are isolated).

❌ Can lead to duplicate logic (each feature implements its own handling).
❌ Harder to maintain cross-cutting concerns (logging, caching, etc.).

✔ Onion/Clean Architecture → Better for long-term projects, domain-driven design, and testability.
✔ Vertical Slicing → Better for microservices, fast iteration, and reducing boilerplate.